Last weekend, I deleted a directory that was "empty" that included some of my personal data.
I use empty with scare quotes because the directory wasn't actually empty. I don't know what exactly caused it, but OS X was listing the directory as empty. I was trying to figure out what was going on with something else, noticed the directory, thought it was some junk directory I never actually used, deleted it, and then a few minutes later that a number of directories were mysteriously empty. I restarted computer and they all came back.
I didn't lose that much data because I do keep most things in git. However, there were a few projects that I had been working on recently that I hadn't pushed remotely.
My personal lesson is to not have things that I want to keep not backed up, no matter how short a time it will be. I know, I know, this is obvious, common advice, but still.